1. Field of the Invention
The present invention relates to monodisperse spherical metal particles and a manufacturing method therefor.
2. Description of the Related Art
In so-called surface mount technology which is technology for mounting electronic devices with a high density on a printed wiring board or the like, a solder paste obtained by admixing solder particles and a paste-like flux is generally used. The solder paste is printed onto a printed wiring board using a screen printing machine, for example, lead terminals of electronic devices are then brought into contact with the top of the applied solder paste, joining is carried out by reflow (heating), and finally precision wiring is formed on the board.
Spherical particles having a diameter of about 20 to 100 μm are predominantly used as the solder particles in the solder paste. In special cases, particles having a diameter of about 10 μm are used. The solder particles are required to have as uniform a particle diameter as possible and as high a sphericity as possible in order to improve and stabilize the printing characteristics. At the same time, it is necessary for the solder particles to prevent as much as possible surface oxidation of the particles, which has a significant influence on solderability.
In addition to solder particles, another spherical solder material called a solder ball which has a diameter of 100 μm to 1 mm is also used as a terminal material for BGA/CSP type packages, which are becoming the most common type of high-density semiconductor packages. As with solder particles, solder balls are required in terms of product quality to have an extremely narrow particle diameter distribution and a high sphericity.
In recent years, there has been an increasingly accelerated demand for electronic equipment typified by mobile phones, digital video cameras, notebook-sized personal computers and the like to be made smaller, lighter, and of higher performance. This is accompanied by advances in reduction in size of electronic devices. There are thus strong demands for surface mount technology to cope with higher density mounting than hitherto. Taking semiconductor integrated circuit packages as an example, the number of terminal pins increases year by year as the degree of integration of the packages increases, and now it has reached several hundred pins, with the pitch between terminal pins being 0.5 to 0.4 mm. In some cases, a pitch of 0.3 mm has been realized in practical use. This is considered to be the practical limit in surface mount technology at present. Various technical developments and improvements are required to further increase the density of surface mounting.
One of these requirements is to make solder particles smaller, make their particle diameter distribution more uniform, and improve their sphericity. Regarding solder balls, it will be necessary for ultrahigh density, ultra-small packages of the next generation to mount several thousand solder balls per square centimeter. That is, for solder balls, technology for making the particle diameter yet smaller while maintaining an extremely narrow particle diameter distribution and a high sphericity is necessary.
At present, for the manufacture of solder particles, minute particle manufacturing techniques such as (1) a centrifugal atomization method (rotating disc method) and (2) a gas atomization method are used. On the other hand, for the manufacture of solder balls, examples of known methods include (1) a method in which relatively large solder particles obtained through the centrifugal atomization method or the gas atomization method are remelted in alumina powder to make the particles perfectly spherical, (2) a method in which a thin solder wire is accurately cut into small sections and the small sections are then made spherical by remelting in an oil bath, and (3) a method involving dripping drops from a narrow nozzle.
However, the solder particles or solder balls obtained through these methods are polydisperse, and hence a classification process is essential. When trying to manufacture minute spherical solder particles having a high sphericity, the yield is thus extremely low, leading to a loss in productivity. Moreover, even if one tries to obtain minute spherical particles through a classification process while ignoring the low yield, the particles have an increased surface area per unit volume, thereby making surface oxidation of the particles marked. It is thus difficult to manufacture minute spherical solder particles suitable for mounting with an increased density as will be demanded in the future.
Accordingly, it will inevitably become the case that prior art solder particles and balls will no longer be able to cope with surface mounting technology in which the mounting density is becoming increasingly higher, and hence development of new alternative materials is considered to be an urgent task.